Arrangement for increasing the safety against unintentional initiation of socalled low energy detonating cord assemblies



3,371,507 INST UNINTENTIONAL G DETONATING P. O. l. GLSSON NT FORINCREASING THE SAFETY INITIATION OF SO-CALLED LOW ENE CORD ASSEMBLIES 2Sheets-Sheet l ARRANGEME March 5, 1968 Filed Aug, 16, 1966 March 5. 1968p. o. OLssoN 3,371,607

ARRANGEMENT FOR INCREASING THE SAFETY AGAINST UNINTENTIONAL INITIATIONOF SOCALLED LOW ENERGY DETONATING CORD ASSEMBLIES Filed Aug. 16, 1966 2Sheets-Sheet 2 INVEN TOR Olaf l. Olsson atet 3,371,607 Patented Mar. 5,1968 tice 3,371,607 ARRANGEMENT FOR INCREASING THE SAFETY AGAINSTUNINTENTIONAL INITIATION F S0- CALLED LOW ENERGY DETONATING CORDASSEMBLIES Per 0. I. Olsson, Gyttorp, Sweden, assignor to Nitro NobelAB, Gyttorp, Sweden, a company of Sweden Continuation-impart ofapplication Ser. No. 428,660, Jan. 28, 1965. This application Aug. 16,1966, Ser. No. 580,844

3 Claims. (Cl. 102-27) This is a continuation-in-part of my applicationSer. No. 428,660 now abandoned.

This invention pertains to an arrangement for increasing the safetyagainst unintentional initiation of so-called low energy detonating cordassemblies.

Low energy detonating cord assemblies are known in the art (Patent No.3,020,844). The main components of such an assembly is the cord, to oneend of which is attached a booster cap and to the other end of which isattached a blasting cap. The cord consists of a metal tube enclosing acore of explosive with one or more layers of insulating material appliedon the outside of the tube. The booster cap and the blasting cap areeach provided with a metal casing. The blasting cap is either of theinstantaneous action or the delayed action type. The core of explosiveof the cord has a suiiicient thickness to permit the detonation topropagate at a high velocity in the axial direction thereof, whereas inthe radial direction the eiect of the detonation is suiliciently low notto cause initiation even of very sensitive explosives surrounding thecore of explosive to take place as the detonation passes. This isattained by using a very thin core of explosive, generally of themagnitude 0.2 g. of pentaerythritol tetranitrate (PETN) or equivalentper meter of the cord.

The low energy detonatingcord assemblies as described above offer thepossibility of carrying out so-called delay initiation with bottom-holepriming, which could previously only be effected while using electricalblasting caps.

As compared with the latter type of caps, the low energy detonating cordassemblies have the advantage that their use does not involve risks ofunintentional initiation, owing to the influence of such low-tensionvoltage and current sources which are frequently present in the workingplaces, such as galvanic cells, stray currents, wireless waves, or powerlines, because the initiation is started through a detonating impulseand cannot be released directly by an electric current.

A device for increasing the safety against unintentional initiation oflow energy detonating cord assemblies caused by high tensions has beendisclosed in the above mentioned specication. This device comprises anopen-ended metal capsule having a central aperture interposed adjacentthe delay element of a delay blasting cap attached to one end of a cord.

That device is shown diagrammatically in longitudinal section in FIG. lof the annexed drawing.

In this ligure, the reference numeral 1 designates a thin core of theexplosive pentaerythritol tetranitrate (PETN) having a diameter of about0.4 mm. The core is enclosed in a lead tube. 2 with an external diameterof about 1.0 mm., and the lead tube is surrounded by one or more layers3 of insulating material, such as paper or plastic. The core ofexplosive, lead tube and layers of insulating material constitute adetonating cord. Attached to and capping one end of this cord is ablasting cap, in principle consisting of a metal shell 4 containing adetonating charge 5, priming charge 6 and a pyrotechnical delay charge7. Provided between this last-mentioned charge and the cord end is aprotecting metal capsule 8 having one end closed and its cylindricalwall resting against the inner wall of the metal shell 4 and its planeend portion provided with a central aperture, through which thedetonation can pass to the blasting cap.

Attached to and capping the other end of the cord is a so-called boostercap, which serves to transmit, by means of a clamping device (branchsleeve) (not shown), the detonation from a trunk line of detonatingfuse. The booster cap consists of a metal shell 9 having a detonatingcharge 10 enclosed therein.

The denomination low energy detonating cord assembly of course includesreasonable modiiications of the embodiment here described. By way ofexample, the lead of the tube may be replaced by some other ductilemetal, or the core of pentaerythritol tetranitrate enclosed in it may bereplaced by a core of some other explosive.

In connection with the development of the present invention, tests wereaccomplished to establish the sensitivity to discharge of staticelectricity of known low energy detonating cord assemblies provided witha protecting metal capsule. These experiments have proved a very goodsafety at discharges equalling 0.003 pf., 15,000,

volts, which is the test standard measure normally used for staticelectricity.

In carrying out the tests, a condenser was connected via ball gapbetween the two conductor systems lead tube 2 and blasting cap shell 4.The capacitance and voltage of the condenser were variable and it wasdischarged by means ofthe ball gap.

However, when these discharges were extended to considerable highervalues such as could in practice be caused by lightning striking nearthe blasting site, astonishing results were obtained. In this trial, theblasting cap and the booster cap detonated in spite of the jumping sparkbetween the two conductor systems being separated from direct Contactwith the explosives of the cap by the protecting metal capsule 8.Obviously this experience will highly limit the value of the low energydetonating cord system (LEDC-system) and must be considered as a greatdrawback.

This drawback can be elimina-ted through this invention, according towhich special metallic bodies are arranged in a certain way to createelectrically conducting connections between the cap shells and themetallic tube of the cord.

Some embodiments of the invention a-re shown on the accompanyingdrawing, in which FIG. 2 shows diagrammatically one embodiment, and FIG.3, likewise diagrammatically, another embodiment. In these iigures, thesame references are used as tor corresponding parts in FIG. 1.

FIG. 2 shows a preferred embodiment of the invention. A metallic tack orstaple 11 has been inserted radially into the cord, about 1 cm. from itsend surface. Crimps 12 between said staple and end surface are preferredif the insulating layer 3 is of a porous material as paper. The tack orlegs of the staple are just long enough for its points to make contactwith the lead tube of the cord without interrupting its PETN-core. Thehead end of the tack or staple is in contact with the cap shell.

FIG. 3 shows another embodiment of the invention. A metallic staple 13has been inserted axially into the cord, with one leg between the leadtube and the insulating layer and the other leg outside the cord. Whenthe cord is pressed into the cap, the last mentioned leg makes contactwith the inside wall of the cap shell, creating good contact pressures,staple to shell and staple to lead tube. In this embodiment of theinvention it is important that the staple is made of a metal with highconductivity, preferably copper, and of a thick wire. In theinvestigated example the staple was made of a copper wire having adiameter of 0.8 mm., and the legs were about 8 mm. and mm. respectively.

FIGURE 4 shows another embodiment of the invention as applied to thebooster cap portion of a low energy cord assembly. As illustrated inFIGURE 4, a metal shell 9 is crimped to insulating material 3 covering alead tube 2,

containing cord 1. A detonating charge is enclosed by the metal shell 9to form a booster cap assembly as On the other hand, if the blasting capis of the delay type and thus contains a pyrotechnical delay-producingcharge, the protecting capsule will have another function in that itdamps the shock wave from the detonating core so as to provide for asuitable `retardation thereof, so as to secure that the pyrotechnicalcombustion is started.

The good technical effect of the invention will appear clearly from thefollowing tests records,

Condenser Estimated Result, current relative Test object Voltage, v.Capacitance, impulse, detonation pf. .Ms frequency,

percent LOW energy detonating cord assemblies With- 1g' (1)(03 "5""- gout arrangement according to the invention 5 000 0' 25 8 10 but withprotecting capsule (8). 5; 000 o: 50 16 100 Low energy detonating cordassemblies with device according to the invention:

(a) Axial copper staple as per FIG. 3....." 5,000 600 0 (b) Radialstaple as per FIG. 2 5,000 40 *3, 000 0 *This test was carried out withalower impedance in the discharge circuit.

shown at the top of FIGURE 1 of the drawings. Further, a metal tack orstaple 11 is positioned through the insulation 3 to provide electricalContact between shell 9 and tube 2.

In the experiments which led to this invention, it was first tried tofit suitably shaped metallic bodies at the top in the boosteranddetonating caps, the bodies making contact with the cap shells and withthe end surfaces of the lead tube, when the cord was pressed into thecaps and fastened with crimps. This device proved, however,

to be quite ineffective in preventing detonation in the discharge testsat extended current impulses mentioned above.

It was not until the contact surface between the lead tube and theconnecting metal body was located inside the cord, as in the embodimentsmentioned above, that the experiments were successful. Then it was infact possible to extend the current impulse of the discharges to valuesexceeding the melting impulse of the lead tube, which is about 600 A2s,without any detonation to occur, Indeed at the limit of the dischargeapparatus used in the experiments, about 3000 A25, no detonationoccurred with the embodiment according to FIG. 2.

When assemblies subjected to such discharges are afterwards dissected,it is visible how the molten lead is pressed out into the insulatingpaper layer. The part of the lead tube which is between the staple pointand the end of the cord is intact, however, and its core of PETN has notdetonated. It is important that this distance is sufcient so that themolten lead is not pressed to the end surface of the cord and ejected tothe heat sensitive charge in the blasting cap. In the investigated cord,whose insulating layer was made of paper cords, about 1 cm. wassuicient, but with other, denser insulating materials this distancecould be substantially reduced.

The reason why a contact at the end surface of the lead tube is not ableto prevent detonation, even at current impulses below the meltingimpulse of the lead tube, is not so easily demonstrated, but may dependon arcing, which is hard to avoid at such extreme current strengths. Forthis reason, it is important that in the embodiment of the inventionaccording to FIG. 2, there is no metallic body in contact, or nearcontact, with the end surface of the lead tube, through which part ofthe discharge current could pass and cause arcing which ejects melteddroplets into the heat-sensitive charge in the blasting cap.

A low energy detonating cord assembly having the a1'- rangementaccording to the invention may be equipped with a protecting capsule ina known manner, although this is not necessary. As a protection againstunintentional initiation the capsule is, on the Whole, unnecessary.

A comparison of the test results for cord assemblies with thearrangement according to the invention and without such an arrangementproves that the improvement concerning safety against the effect of hightension discharges amounts to at least one centifactor.

In the embodiments shown, the arrangement according to the invention hasnot been introduced in the booster cap because corresponding dischargetests have not resulted in detonations. However, it is fully possible todo so without exceeding the scope of the invention.

What I claim is:

1. A low energy detonating cord assembly comprising a detonating cord,including, a central, elongated core of high velocity, detonatingexplosive, a metallic annular tube immediately surrounding said core, atleast one annular layer of insulating material immediately surround ingsaid metallic tube; a booster cap, including, a cap-type, metal exteriorshell having one end closed and its opposite end at least partiallysurrounding and in contact with said insulating layer at one end of saiddetonating cord; a

blasting cap, including, a cap-type metal exterior shell having one endclosed and its opposite end at least partially surrounding and incontact with said insulating layer at the other end of said detonatingcord; a wirelike electrical conducting means, electrically connectingthe outer lateral wall of said anular tube to the inner lateral wall ofat least one of said blasting cap shell and said booster cap shell andpassing through a substantial portion of said insulation, wherebydetonation of said assemble by stray electrical current is substantiallyreduced.

2. An assembly as in claim 1 with an electrical conducting means,electrically connecting the outer lateral wall of said annular tube tothe inner lateral wall of said booster cap shell, whereby detonation ofsaid assembly by stray electrical currents is substantially reduced.

3. An assembly as in claim 1 with an electrical conducting means,electrically connecting the outer lateral wall of said annular tube tothe inner lateral wall of said blasting cap shell, whereby detonation ofsaid assembly by stray electrical currents is substantially reduced.

References Cited UNITED STATES PATENTS 2,424,374 7/ 1947 Taylor et al.102-27 3,106,892 10/1963 Miller 102-27 3,129,663 4/1964 Schnepfe 102-27BENJAMIN A. BORCHELT, Primary Examiner.

V. R. PENDEGRASS, Assistant Examiner.

1. A LOW ENERGY DETONATING CORD ASSEMBLY COMPRISING A DETONATING CORD,INCLUDING, A CENTRAL, ELONGATED CORE OF HIGH VELOCITY, DETONATINGEXPLOSIVE, A METALLIC ANNULAR TUBE IMMEDIATELY SURROUNDING SAID CORE, ATLEAST ONE ANNULAR LAYER OF INSULATING MATERIAL IMMEDIATELY SURROUNDINGSAID METALLIC TUBE; A BOOSTER CAP, INCLUDING, A CAP-TYPE, METAL EXTERIORSHELL HAVING ONE END CLOSED AND ITS OPPOSITE END AT LEAST PARTIALLYSURROUNDING AND IN CONTACT WITH SAID INSULATING LAYER AT ONE END OF SAIDDETONATING CORD; A BLASTING CAP, INCLUDING A CAP-TYPE METAL EXTERIORSHELL HAVING ONE END CLOSED AND ITS OPPOSITE END AT LEAST PARTIALLYSURROUNDING AND IN CONTACT WITH SAID INSULATING LAYER AT THE OTHER ENDOF SAID DETONATING CORD; A WIRELIKE ELECTRICAL CONDUCTING MEANS,ELECTRICALLY CONNECTING THE OUTER LATERAL WALL OF SAID ANNULAR TUBE TOTHE INNER LATERAL WALL OF AT LEAST ONE OF SAID BLASTING CAP SHELL ANDSAID BOOSTER CAP SHELL AND PASSING THROUGH A SUBSTANTIAL PORTION OF SAIDINSULATION, WHEREBY DETONATION OF SAID